£600 to replace 1 injector is very steep. All 4 should be replaced because, as you say, it's a beefy job labour wise. You don't want to have to repeat the work when injectors 1 & 2 fail.
Say £300 for injectors + 2-3 hours labour, should bring you out at the £500 mark.....but all 4 will be done. May as well chuck in another 100 whilst you're there for a walnut blast. That's indie pricing. Stealership will be a grand I reckon.
Edit: the dealer price is steep because the injectors are £100+VAT each, + 4 x fitting kits at £25+VAT each, plus the labour at £100+VAT p/h. The online injectors come with the teflon seals prefitted, which is a massive cost and time saver.